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What is on the menu for zooplankton?

For 150 years, scientists have been trying to get a proper line on what zooplankton (animal plankton) actually eat. Now an interdisciplinary team of researchers from UiB have discovered a useful method that can provide new knowledge about both the oceanic food chain and the effects of climate change.

The researchers have discovered that it is possible to use a genetic technique, so-called ‘quantitative PCR analysis’, to study the stomach contents of the zooplankton Oikopleura. This analysis method gives the researchers both information on what the animal has eaten, what prey animals it prefers when it has a chance to eat several species, and also how much of each it then eats.

‘This is important knowledge, for zooplankton, such as Oikopleura, are among the most numerous multicellular organisms in the ocean. And they play a key role,’ says researcher Jens Nejstgaard, a marine biologist and expert on zooplankton.

‘Impressive adaptability’

Zooplankton are important in the food chain, they are probably more important in sedimentation processes than we used to think, and they contribute to regulating the population of both plant plankton and smaller zooplankton.

‘They can both limit and reinforce undesirable consequences of human impact on the environment,’ explains Nejstgaard.

It is precisely this adaptability, and the fact that zooplankton is a selective feeder when they get the chance, that has been a cause of frustration for the researchers. In addition, they can move over a wide area, which has made it difficult to find out what they actually eat in an undisturbed environment.

‘This very question has gone unanswered for 150 years. We have not yet found the answer to it, but now we have shown that we have a usable method,’ emphasises Troedsson.

Previous methods – one of which was to examine the stomach contents of zooplankton under the microscope – had major weaknesses.

‘If rice is all you are given, you eat rice’

‘But here we are talking about an organism that is up to a couple of millimetres long, eating other organisms that may be less than a micrometre. Firstly, analysing them is very time-consuming, and secondly, it is rarely easy to identify more than a fraction of the prey animals, as they dissolve very rapidly’ explains Nejstgaard.

Another method was therefore to put the organisms into a bottle with a known quantity of other, smaller plankton, wait a while, and see what is left in the bottle.

‘The problem is just that we are taking a small sample of a large, complex and heterogeneous environment. It does not necessarily correspond to the environment that the plankton are experiencing in reality,’ Nejstgaard continues. ‘If you are in prison, and rice is all you are given, then you eat rice. That does not mean that you prefer it after you are released.’

Brilliant genetics

There is another problem with Oikopleura. The animal itself is not very big, but it is surrounded by a big mucus house that catches other organisms from the ambient water; this mucus house is then discarded, and a new one is grown, between four and 20 times a day. This has made the job harder for the researchers and led to mistaken conclusions. For just because the animal catches other organisms in its mucus house, that doesn’t mean it eats them.

That is why the DNA analysis from the interdisciplinary UiB team is such an advance. It means that the researchers gain a complete overview of what the organism has in its digestive apparatus, even if the prey animals cannot be recognised by their appearance. By comparing this to what is caught in the mucus house or is found in the water around the organism, the researchers can also find out what prey Oikopleura prefers.

‘And not least, we can find out what they eat in situ, that is where we catch them, not what they choose to eat from the known contents of a bottle,’ explains Eric Thompson from the Sars Centre. Thompson, the team’s molecular development biologist, has spent several research years establishing Oikopleura as a model organism and building up world-class laboratory facilities at the Sars Centre. It is there that the UiB researchers, together with the American expert Marc Frischer, have succeeded in experimenting their way to this method.

Want to get out into the field

Thompson describes how the scientists first analyse everything that is in the DNA profile of the contents of Oikopleura’s stomach, then determine what species the DNA fragments come from. In this way they can also discover whether Oikopleura eats species that the researchers are unable to cultivate in the laboratory – something that is by no means easy to achieve in the bottle experiments.

‘Over time the DNA will be dissolved in the stomach contents,’ explains Thompson, ‘but Christofer Troedsson developed a method that can extrapolate backwards, so that we can find out what it ate when.’

The first results are from the testing of the method in the laboratory, and have just been published in ‘Limnology and Oceanography’. Now the researchers want to get out into the field.

‘One of the benefits of our having such a broad, interdisciplinary ballast in this team is that we can refine this method step by step: first under highly controlled conditions in the laboratory, then under mesocosmic conditions, on which Nejstgaard has done a lot of work,’ explains Thompson.

‘Too little research onzooplankton’

The team is not only hoping to learn more about the role of zooplankton, in the long run they envisage that such a method can also have consequences for research into both fisheries and climate. The latter because there is reason to believe that organisms such as Oikopleura may play a much greater role in sedimentation processes than has so far been assumed.

It is namely an argument in the climate debate that an increased concentration of CO2 in the atmosphere will lead to acidification of the oceans. If the pH level in the ocean falls, it will go particularly hard with the small organisms with a calcium shell. Many such organisms are important to the vertical transport of carbon in the oceans, they bind carbon from the surface and take it to the seabed, and this will not happen if they dissolve on the way down.

The Oikopleura mucus house, however, also contributes to these accounts, and is not affected by acidification. For this reason some scientists believe that the relative importance of different carbon transport routes in the ocean can change, and that particle-filled Oikopleura mucus houses may thus be assigned a more important role in the process.

‘But we do not yet know how important organisms without such a siliceous shell like Oikopleura are in these accounts. The role of zooplankton in general is insufficiently studied. This method, however, gives us a valuable tool that may provide greater insight into how these complicated biological systems work,’ says Nejstgaard.